The objective of this study was to assess overall vaccine initiation and completion in the active component U.S. military, with a focus on racial/ethnic disparities. From 11 December 2020 through 12 March 2021, a total of 361,538 service members (27.2%) initiated a COVID-19 mRNA vaccine. Non-Hispanic Blacks were 28% less likely to initiate vaccination (95% confidence interval: 25%-29%) in comparison to non-Hispanic Whites, after adjusting for potential confounders. Increasing age, higher education levels, higher rank, and Asian/Pacific Islander race/ethnicity were also associated with increasing incidence of initiation after adjustment. When the analysis was restricted to active component health care personnel, similar patterns were seen. Overall, 93.8% of those who initiated the vaccine series completed it during the study period, and only minor differences in completion rates were noted among the demographic subgroups. This study suggests additional factors, such as vaccine hesitancy, influence COVID-19 vaccination choices in the U.S. military. Military leadership and vaccine planners should be knowledgeable about and aware of the disparities in vaccine series initiation.